The present invention generally relates to an alkali metal electrochemical cell, and more particularly, to a rechargeable alkali metal cell. Still more particularly, the present invention relates to a lithium ion electrochemical cell activated with an electrolyte having an additive provided to achieve high charge/discharge capacity, long cycle life and to minimize the first cycle irreversible capacity. According to the present invention, the preferred additive to the activating electrolyte is a phosphonate compound. A phosphite compound is another name for a phosphonate compound and is also preferred for the present invention.
Alkali metal rechargeable cells typically comprise a carbonaceous anode electrode and a lithiated cathode electrode. Due to the high potential of the cathode material (up to 4.3 V vs. Li/Li+ for Li1-xCoO2) and the low potential of the carbonaceous anode material (0.01 V vs. Li/Li+ for graphite) in a fully charged lithium ion cell, the choice of the electrolyte solvent system is limited. Since carbonate solvents have high oxidative stability toward typically used lithiated cathode materials and good kinetic stability toward carbonaceous anode materials, they are generally used in lithium ion cell electrolytes. To achieve optimum cell performance (high rate capability and long cycle life), solvent systems containing a mixture of a cyclic carbonate (high dielectric constant solvent) and a linear carbonate (low viscosity solvent) are typically used in commercial secondary cells. Cells with carbonate based electrolytes are known to deliver more than 1,000 charge/discharge cycles at room temperature.
One aspect of the present invention involved the provision of ethylene carbonate (EC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC) and diethyl carbonate (DEC) as the solvent system for the activating electrolyte. Lithium ion cells with such electrolyte systems are capable of discharge at temperatures down to as low as xe2x88x9240xc2x0 C. while exhibiting good cycling characteristics. However, lithium ion cell design generally involves a trade off in one area for a necessary improvement in another. The achievement of a lithium-ion cell capable of low temperature cycleability by use of the above quaternary solvent electrolyte, in place of a typically used binary solvent electrolyte (such as 1.0 M LiPF6/EC:DMC=30:70, v/v which freezes at xe2x88x9211xc2x0 C.), is obtained at the expense of increased first cycle irreversible capacity during the initial charging (approximately 65 mAh/g graphite for 1.0 M LiPF6/EC:DMC:EMC:DEC=45:22:24.8:8.2 vs. 35 mAh/g graphite for 1.0 M LiPF6/EC:DMC=30:70). Due to the existence of this first cycle irreversible capacity, lithium ion cells are generally cathode limited. Since all of the lithium ions, which shuttle between the anode and the cathode during charging and discharging originally come from the lithiated cathode, the larger the first cycle irreversible capacity, the lower the cell capacity in subsequent cycles and the lower the cell efficiency. Thus, it is desirable to minimize or even eliminate the first cycle irreversible capacity in lithium ion cells while at the same time maintaining the low temperature cycling capability of such cells.
According to the present invention, these objectives are achieved by providing an organic phosphonate or phosphite in the quaternary solvent electrolyte. Lithium ion cells activated with these electrolytes exhibit lower first cycle irreversible capacities relative to cells activated with the same quaternary solvent electrolyte devoid of the phosphonate additive. As a result, cells including the phosphonate additive present higher subsequent cycling capacity than control cells. The cycleability of the present invention cells at room temperature, as well as at low temperatures, i.e., down to about xe2x88x9240xc2x0 C., is as good as cells activated with the quaternary electrolyte devoid of a phosphonate additive.
It is commonly known that when an electrical potential is initially applied to lithium ion cells constructed with a carbon anode in a discharged condition to charge the cell, some permanent capacity loss occurs due to the anode surface passivation film formation. This permanent capacity loss is called first cycle irreversible capacity. The film formation process, however, is highly dependent on the reactivity of the electrolyte components at the cell charging potentials. The electrochemical properties of the passivation film are also dependent on the chemical composition of the surface film.
The formation of a surface film is unavoidable for alkali metal systems, and in particular, lithium metal anodes, and lithium intercalated carbon anodes due to the relatively low potential and high reactivity of lithium toward organic electrolytes. The ideal surface film, known as the solid-electrolyte interphase (SEI), should be electrically insulating and tonically conducting. While most alkali metal, and in particular, lithium electrochemical systems meet the first requirement, the second requirement is difficult to achieve. The resistance of these films is not negligible, and as a result, impedance builds up inside the cell due to this surface layer formation which induces unacceptable polarization during the charge and discharge of the lithium ion cell. On the other hand, if the SEI film is electrically conductive, the electrolyte decomposition reaction on the anode surface does not stop due to the low potential of the lithiated carbon electrode.
Hence, the composition of the electrolyte has a significant influence on the discharge efficiency of alkali metal systems, and particularly the permanent capacity loss in secondary cells. For example, when 1.0 M LiPF6/EC:DMC=30:70 is used to activate a secondary cell, the first cycle irreversible capacity is approximately 35 mAh/g of graphite. However, under the same cycling conditions, the first cycle irreversible capacity is found to be approximately 65 mAh/g of graphite when 1.0 M LiPF6/EC:DMC:EMC:DEC=45:22:24.8:8.2 is used as the electrolyte. In contrast, lithium ion cells activated with the binary solvent electrolyte of ethylene carbonate and dimethyl carbonate cannot be cycled at temperatures less than about xe2x88x9211xc2x0 C. The quaternary solvent electrolyte of the previously referenced patent application, which enables lithium ion cells to cycle at much lower temperatures, is a compromise in terms of providing a wider temperature application with acceptable cycling efficiencies. It would be highly desirable to retain the benefits of a lithium ion cell capable of operating at temperatures down to as low as about xe2x88x9240xc2x0 C. while minimizing the first cycle irreversible capacity.
According to the present invention, this objectives is achieved by adding a phosphonate additive in the above described quaternary solvent electrolytes. In addition, this invention may be generalized to other nonaqueous organic electrolyte systems, such as binary solvent and ternary solvent systems, as well as the electrolyte systems containing solvents other than mixtures of linear or cyclic carbonates. For example, linear or cyclic ethers or esters may also be included as electrolyte components. Although the exact reason for the observed improvement is not clear, it is hypothesized that the phosphonate additive competes with the existing electrolyte components to react on the carbon anode surface during initial lithiation to form a beneficial SEI film. The thusly formed SEI film is electrically more insulating than the film formed without the phosphonate additive and, as a consequence, the lithiated carbon electrode is better protected from reactions with other electrolyte components. Therefore, lower first cycle irreversible capacity is obtained.
These and other objects of the present invention will become increasingly more apparent to those skilled in the art by reference to the following description.